1. Field of Invention
Various embodiments of the present invention relate to optical communication, and more particularly, to a tunable laser for use in optical communication.
2. Description of Related Art
The term ‘LASER’ is an acronym for ‘Light Amplification by Stimulated Emission of Radiation’, and the main element necessary for the formation of a laser is a resonator that includes a gain medium and a pump that supplies energy to the laser. For a radiated light to be amplified by a stimulated emission process, there must be a suitable gain medium the type of which determines the oscillation wavelength band of the laser. The most representative examples of lasers that are relatively easily tunable are gas lasers, dye lasers, and solid-state lasers, according to the types of the gain medium.
However, in the case of such a general resonator based laser, the wavelength band is limited to an emission bandwidth that the gain medium has, and thus a nonlinear frequency conversion method is frequently used as a way to overcome this. By transmitting light through a suitable nonlinear single crystal in accordance with a phase matching condition and then using an SFG (sun frequency generation) method, DFG (difference frequency generation) method, and an OPG & OPA (optical parametric generation & amplification) method, it is possible to expand a tuning range from an ultraviolet ray to infrared area without preparing an additional resonator. Besides the aforementioned, it is also possible to configure an additional resonator including a nonlinear single crystal such as an OPO (optical parametric oscillator) and resonating a light generated by an optical medium effect within the resonator, thereby expanding the wavelength adjustment range.
Research is actively underway on passive optical networks (PON) that are based on wavelength division multiplexing (WDM) (hereinafter referred to as ‘WDM-PON’). These networks have their basis on adjusting wavelengths. WDM-PON may provide converged services of voice, data, and broadcast.
In WDM-PON, each subscriber communicates with a center office (CO) using a different wavelength assigned to the subscriber. And since each subscriber uses an exclusive wavelength assigned to each subscriber, WDM-PON has excellent security and enables providing massive communication services, and thus it has an advantage that each subscriber or service may be provided with a transmission technology of a different link rate and frame format.
However, since WDM-PON is a technology of multiplexing various wavelengths in a single optical fiber using the WDM technology, it requires different light sources as many as the number of subscribers that belong to one remote node (RN). Production, installation, and management of light source per wavelength are becoming a great burden to users and subscribers, and thus a big obstacle to commercialization of WDM-PON. In order to resolve such a problem, application methods of tunable lasers capable of selectively tuning the wavelength of a light source is actively being studied.